Related Rates

A swimming pool is 25 ft wide, 60 ft long, 3 ft deep at the shallow end, and 15
ft at its deepest point. If the pool is being filled at a rate of 800 ft^3/min,
at what rate is the water level rising when the depth at the deepest point is
5ft.

I'm stuck in this exercise. I just need a tip for the exercise. Thanks for you
help.

I did this:
Let x feet be the wide, and y the long, t for time measured in minutes.

dx/dt= 25 ft and I need to find dy/dt when y = 60ft.

I know that I have to write an equation that relates the various quantities of
the problem. But I don't know what equation I should put.

A swimming pool is 25 ft wide, 60 ft long, 3 ft deep at the shallow end, and 15 ft at its deepest point.
If the pool is being filled at a rate of 800 ft³/min, at what rate is the water level rising
when the depth at the deepest point is 5ft?